Embodiments of the present invention relate to an apparatus for amplifying an input-signal. Some embodiments relate to an apparatus for amplifying an input-signal, the apparatus comprising a switch-mode amplifier for amplifying a digital input-signal and a generator for generating the digital input-signal. Some embodiments relate to a vector modulating linear switch-mode amplifier.
In electronics and especially in wireless communication there are many applications where bandpass-signals have to be generated and amplified. A bandpass-signal is an electrical signal, whose spectral energy is limited to a certain bandwidth around a carrier frequency. The bandpass-signal has no DC component and no spectral components above a certain cut-off frequency. The bandwidth is typically several percent of the carrier frequency. In most applications, a bandpass-signal is generated by means of digital signal-processing, whereas the signals are represented as complex-valued digital baseband-signals. A complex-valued baseband-signal has two components. A real and an imaginary part, respectively an I- and a Q-component. The common procedure is, to convert the digital I- and Q-signals to real-valued analog low-pass-signals and to bring them into the bandpass-domain, using an IQ-Mixer or vector modulator, driven by a harmonic signal of carrier frequency. Therefore, the IQ-Mixer can be seen as a frequency converter and its operation as frequency conversion. In general, a bandpass-signal has a non-constant envelope, characterized by the peak-to-average power ratio. In many cases, the bandpass-signal has to be amplified by means of an amplifying device.
A power amplifier is a two port device which has a port for the input-signal and a port for the output-signal. It uses an auxiliary power source to produce an output-signal with increased power compared to the input-signal. An amplifier is realized by means of amplifying devices such as transistors or tubes. These amplifying elements are nonlinear in general. In most technical applications, e.g. in wireless communication, a nonlinear distortion of the output-signal has to be avoided, since it produces unwanted out-of-band emissions and in-band distortions. A nearly perfect linear behavior is accomplished by driving the amplifying element with small signal amplitudes, compared to the maximum amplitude allowed by the device. Increasing the input amplitude results in a more nonlinear behavior, where the highest signal values in the output-signal become slightly compressed. But the output-signal has still a varying magnitude and the amplifier is denoted to be weakly nonlinear. In contrast to that, a strongly nonlinear behavior results in a hard bounded amplitude of the output-signal which is constant and independent from the input magnitude. Such a signal has only two states, which correspond to the signum function of the input-signal, and will be referred to as a binary signal in the following.
Examples for amplifiers showing almost no or weak nonlinearity are class A or class AB amplifiers, examples for strong nonlinear amplifiers are class D and class E amplifiers such as digital line drivers and pulse amplifiers. Those strong nonlinear amplifiers will be denoted as switch-mode amplifiers in the following, since the amplifier basically acts like a switched current source, which is triggered by the signum function of the input-signal.
The efficiency of an amplifier is defined as the ratio of the average power of the output-signal to the input power, where the input power is the average power delivered from the auxiliary power source added to the power of the driving input-signal. The efficiency of a weak nonlinear amplifier is comparatively low and decreases even more for signals with a high peak to average power ratio. In contrast to that, switch-mode amplifiers show an efficiency of up to 1, which is based on the fact, that ideally either the voltage across or the current through the switching element is zero at any time instant. Therefore, from the efficiency point of view, a switch-mode amplifier is highly desirable. On the downside a switch-mode amplifier removes any amplitude information from the input-signal by definition. Therefore, the amplitude of the input-signal has to be preserved by the additional application of a pulse modulation scheme at the input of the amplifier which results in a binary input-signal, with all information encoded in the zero crossings of the signal. At the output an additional demodulation of the amplified binary signal is needed to reconstruct the original signal. Moreover, the modulation produces unwanted out-of-band spectral components which have to be suppressed by the demodulation procedure. Otherwise the whole system—modulator, switch-mode amplifier, demodulator—would not behave like a linear power amplifier by definition.
To accomplish an undistorted i.e. linear amplification of the signal envelope, the amplifying device has to be operated with a certain headroom i.e. power back-off, which decreases the amplifier power-efficiency. Otherwise, the amplifying device may deform the bandpass-signal non-linearly, producing unwanted in-band distortions and unwanted out-of-band emissions. The poor power-efficiency of amplifiers for bandpass-signals, which may be as low as 10 percent for modern wireless communication signals causes a high demand for new amplifier concepts. Current approaches, which use adaptive digital amplifier predistortion show a massively increased hardware effort. Therefore there is also a wish for integrated solutions with reduced hardware complexity.
There are several solutions known to increase the power efficiency for bandpass-amplifiers. On circuit level, the Doherty topology provides better efficiency for high power back-off values. Envelope Elimination and Restoration and Bias Modulation are other techniques on system level to increase efficiency in the back-off operating region. Analogue feed-forward and feed-back circuits may be used to improve linearity for lower power-back-offs, which also increases the power efficiency. In contrast to this, closed-loop digital predistortion is implemented on system level and shows better linearity improvements for signals with comparatively large bandwidth. It is also capable to adapt to temporal changes in the amplifying device.
Whereas the mentioned solutions are designed for the improvement of amplifiers, which are linear in principle, the switch-mode techniques are intended to transform the bandpass-signal in a sequence of rectangular, binary pulses, which could be amplified with an theoretical efficiency of 100 percent. After amplification, the original bandpass-signal is reconstructed by low-pass filtering. From an engineering perspective, the transformation and reconstruction can be seen as a pulse modulation and demodulation problem. One well known switch-mode amplifier technique uses pulse width modulation (PWM), which is a feed-forward modulation scheme. Another well known switch-mode amplifier technique uses sigma delta modulation (SDM), which is a closed loop modulation scheme. Both schemes have the disadvantage that they produce inband signal distortions which can not be removed by the demodulator. These inband signal distortions can be reduced by increasing the switching frequency, by oversampling or by higher order loopback filtering (for SDM). This increases the demands on the amplifying device and therefore the cost of switch-mode amplifiers, especially for operation at high signal frequencies which are typical for radio frequency (RF) and microwave applications. Moreover, the closed loop architecture of the sigma delta modulation (SDM) concept tends to instability problems at very high operational frequencies because of feedback delay.